why so juicy - technial bods

[dan4]

Why are rotaries renound for their excessive fuel consumption?

From various posts i have noticed that you cruise on stoich AFR\'s and full throttle AFR\'s range from 10.5 to 11, my supra cruises on stoich and at full throttle is 11.5 so pretty similar. So my query is that if a car uses a set amount of air it requires a set amount of fuel to hit target AFR, double the air flow requires doube the fuel flow there if rotaries use similar AFR\'s to piston engines where does the increased consumption come from?

[Prof]

Why are rotaries renound for their excessive fuel consumption?
 
From various posts i have noticed that you cruise on stoich AFR\'s and full throttle AFR\'s range from 10.5 to 11, my supra cruises on stoich and at full throttle is 11.5 so pretty similar. So my query is that if a car uses a set amount of air it requires a set amount of fuel to hit target AFR, double the air flow requires doube the fuel flow there if rotaries use similar AFR\'s to piston engines where does the increased consumption come from?

They run considerably richer on idle and are very rich on startup - so that accounts for a small percentage of the fuel used. Although we theoritcally get stoich or close to at cruise - that\'s only for the cars that still have the lambda sensor in and run closed loop - and who are actually driving around at cruise RPM\'s - for 2nd Gens that\'s between 1500-3500 RPM off boost - but that rarely happens :rollin
 
For most of the FD guys running Apexi or other standalone - they are probably running open loop and have mapped the car to be close to 14 AFR on cruise - but likelyhood is it\'s on the rich side for safety.
 
Driven in the same manor as an equivalent piston engined car of comparable power - they aren\'t that far off in terms of fuel consumption (my wifes 300zx does about the same MPG on an average run)

[buckman67]

Driven in the same manor as an equivalent piston engined car of comparable power - they aren\'t that far off in terms of fuel consumption (my wifes 300zx does about the same MPG on an average run)

Yeh I agree with that.  Had an S2000 for a number of years. Has been car of the year and gets great praise from the motoring press and motoring fans, mainly because its a great car, but driven with gusto you be lucky to get 20mpg and generally at pace you\'ll get 15 - 16 mpg.

Rotaries generally use above average fuel because they are very very very fast :yes:yes

[Brett]

A large proportion of the \'excess\' fuel consumption on the rotaries is a by product of the emissions control system.  Rotaries run an air pump to reintroduce oxygen into the exhaust flow, thusly causing a reburn of the exhaust gases and reducing the amount of unburnt hydrocarbons.  In order to get a full burn Mazda discovered that they actually needed to intorduce more fuel into the equation to make the system work efficiently.

I\'m not sure if this is still the case on the later rotaries, but it affected the pre Rx8 cars.

:wave

[David Nock (RIP)]

Plus I\'m sure I read somewhere that mechanically the rotary motion is superior to a reciprocating engine but that in turn brings with it the problem of the combustion chamber shape - elongated and far too long for an efficient combustion flame at high revs. That\'s why there are 2 plugs (lead/lagging), sparking as the rotor spins past, but still not all the fuel gets fully combusted.

[dan4]

A large proportion of the \'excess\' fuel consumption on the rotaries is a by product of the emissions control system.  Rotaries run an air pump to reintroduce oxygen into the exhaust flow, thusly causing a reburn of the exhaust gases and reducing the amount of unburnt hydrocarbons.  In order to get a full burn Mazda discovered that they actually needed to intorduce more fuel into the equation to make the system work efficiently.

I\'m not sure if this is still the case on the later rotaries, but it affected the pre Rx8 cars.

:wave

but again dude if its running at stoich then where is the extra fuel......

They run considerably richer on idle and are very rich on startup

Like what AFR\'s? All piston engines use start enrichment decay and coolant enrichment so start rich, its just a way of ensuring smooth running and ensuring good burn in poor situations


Im intrigued by it all as lets say a FD engine is the equivalent of 2.6(correct me if im wrong!) and my supra is a 3.0, the FD should be lowing less air and therefore less fuel cruising at stoich or making full throttle at 11.0AFR at a given rpm?

[Prof]

but again dude if its running at stoich then where is the extra fuel......

If it is running Stoich then there won\'t be any extra fuel.
 
 
 

Like what AFR\'s? All piston engines use start enrichment decay and coolant enrichment so start rich, its just a way of ensuring smooth running and ensuring good burn in poor situations?

You can expect below 10:1 on initial startup, slackening off to around 12:1 - 13:1 - all depends a lot on porting and what have you.  Standard engines can be made to idle at 700-900 RPM, with porting, this idle will increase - a Bridge Ported engine will idle at 1500 RPM - so straight away more fuel is being used just to hold the car stationary and have the engine running.
 
 

Im intrigued by it all as lets say a FD engine is the equivalent of 2.6(correct me if im wrong!) and my supra is a 3.0, the FD should be lowing less air and therefore less fuel cruising at stoich or making full throttle at 11.0AFR at a given rpm?

Take crusing first - the standard FD is a twin turbo - the first turbo will spool at just over 2000 RPM, so you could be getting some boost even at mid RPM\'s - this will effect economy.  The standard ECU does have closed loop - so will adjust fuelling within the relatively small RPM range that the car will \'cruise\'.  A lot of FD\'s are far from standard - they have aftermarket ECU\'s, single Turbos, twin turbo\'s reworked to operate non sequentially - all these things can give more power, but at the expense of cruising with good AFR\'s.  Good mapping can get close to Stoich - but a lot of people run open loop so the lambda isn\'t used.
 
At WOT - the AFR\'s will depend on porting, timing - and the cooling setup of the car - AFR of 11 is a good target for WOT - it may be lower - more like 10 on many cars.
 
Are you looking for a comparison against a Supra ?  Are we talking N/A or Turbo ?  Is it stock power or does it have upgrades ?
 
Might be worth putting up some data and having someone with an FD of similar power levels putting up their AFR and economy figures.

[noyzboy]

If it is running Stoich then there won\'t be any extra fuel.
 
 
 

 
You can expect below 10:1 on initial startup, slackening off to around 12:1 - 13:1 - all depends a lot on porting and what have you.  Standard engines can be made to idle at 700-900 RPM, with porting, this idle will increase - a Bridge Ported engine will idle at 1500 RPM - so straight away more fuel is being used just to hold the car stationary and have the engine running.
 
 

 
Take crusing first - the standard FD is a twin turbo - the first turbo will spool at just over 2000 RPM, so you could be getting some boost even at mid RPM\'s - this will effect economy.  The standard ECU does have closed loop - so will adjust fuelling within the relatively small RPM range that the car will \'cruise\'.  A lot of FD\'s are far from standard - they have aftermarket ECU\'s, single Turbos, twin turbo\'s reworked to operate non sequentially - all these things can give more power, but at the expense of cruising with good AFR\'s.  Good mapping can get close to Stoich - but a lot of people run open loop so the lambda isn\'t used.
 
At WOT - the AFR\'s will depend on porting, timing - and the cooling setup of the car - AFR of 11 is a good target for WOT - it may be lower - more like 10 on many cars.
 
Are you looking for a comparison against a Supra ?  Are we talking N/A or Turbo ?  Is it stock power or does it have upgrades ?
 
Might be worth putting up some data and having someone with an FD of similar power levels putting up their AFR and economy figures.

FDs don\'t do "economy"  LOL

Martin H

[Timmy]

I just did a long motorway run and some r/r time at 19mpg now i think i am right in thinking that is about the same as a supra TT.

[dan4]

im not trying to consider a car thats running ported/heavily tuned to be honest i was more trying to consider the stock fuel economy. Im not comparing it in a way of supra vs rx7 i just used my supra as an example as i had good proof of AFR\'s rather than just using guess work to compare

Just because an FD makes boost at 2k doesnt mean its not in closed loop, supra tt\'s/300zx\'s etc make boost that low also. Closed loop on a supra is approx 60% tps up to 4k rpm providing coolant temp is 60deg, this of course varies car to car. Maybe someone knows exactly defines closed loop on rx7

My supra has a big single currently over 500hp and will return almost 25mpg on a run no prob, obviously without nailing the loud pedal. As i said i only use it as comparison as i know exactly what AFRs occur where etc....

My WOT is 11.5 and falls to 10.8 towards rev limit

Im just trying to work out the reason for rotaries being blamed for poor economy, seems like a good debate started:cheers:cheers:cheers

FDs don\'t do "economy" LOL

Martin H

thats good but doesnt really answer question:flamed

I just did a long motorway run and some r/r time at 19mpg now i think i am right in thinking that is about the same as a supra TT

60litres over approx 320miles equates to approx 23mpg which i got driving to and from (was not properly mapped on way up) Surrey RR and including dyno pulls and some road tuning at 500hp so fairly close

[m1tch]

You shouldn\'t talk economy figures on a rotary lol my almost stock FC does between 6 and 17 mpg

[Prof]

The original FD ECU\'s were very similar to the FC ECU. However, the FC uses an AFM to meter air and calculate fuel maps until the car hits boost or higher throttle settings, whereas the FD uses a MAP sensor.
 
Closed loop on the FC is between 1500-3500 RPM off boost and with warm engine (not sure on the exact setting - but the wax thermo pellet on the throttle body isn\'t fully off the throttle cam until the water temps are in the 70\'s. Not sure what the settings are on an FD - but I\'d expect they are similar.
 
Unlike with Piston engines, the Lambda sensor is used a lot less on rotaries - and so fuel correction using the Lambda is over a very limited range.
 
The other fuel correction mechanisms are effectively open loop - as both the AFM and MAP methods fix fuel depending on measured / metered air
 
On gentler runs (icy roads or after a rebuild) so keeping off boost, my car returned upto 25mpg (street porting didn\'t appear to make much difference to economy). Driving like that you are mostly in the closed loop and so running Stoich.
 
On a run to Scotland with plenty of fast motorway / overtaking / full throttle bursts etc. I got 18mpg average - again there would have been some crusing in amongst that for larger periods - but comparable with a piston engined car driven in the same manor with similar power.
 
I think it\'s fair to say that rotaries are a little thirstier than piston engines - but perhaps not as bad as has been painted.

[Prof]

Have a good read of the Wiki article - will probably help explain a few things:
 
http://en.wikipedia.org/wiki/....el_engine

[Cashpoint]

I feel stupid but what is "stoich"?

[buckman67]

I feel stupid but what is "stoich"?

Stoich is an abreviation for Stoichametric. If an engine manages to burn all the fuel inducted rather than chuck some out of the exhaust, the fuel air ratio is said to be Stoichameteric. In normal petrol that is 14.7 - 1,  i.e. if you induct 14.7 lbs of air and 1 lb of petrol into an efficient engine it will burn all the available fuel and no Oxygen will be exhausted.